Differential expression of thyroid-stimulating hormone β subunit in gonads during sex reversal of orange-spotted and red-spotted groupers

Abstract

We have cloned and characterized the full-length cDNA encoding thyroid-stimulating hormone β-subunit (TSHβ) from orange-spotted grouper Epinephelus coioides. It contains 913 nucleotides with an open reading frame encoding 146 amino acids with a 20 amino acid signal peptide. The grouper mature TSHβ has 75, 70, 61, 59, 41, 42 and 40% identities to that of rainbow trout, Atlantic salmon, zebrafish, European eel, chicken, mouse and human, respectively. RT-PCR analysis indicated that the TSHβ mRNA was expressed abundantly not only in pituitary but also in gonads. A more interesting finding is to reveal the differential TSHβ expressions between the ovaries and the transitional gonads or testes in natural individuals of orange-spotted grouper and red-spotted grouper Epinephelus akaara, and in artificial sex reversal individuals of red-spotted grouper induced by MT feeding. In situ hybridization localization provided direct evidence that the TSHβ was transcribed in the germ cells. In the growing oocytes, the TSHβ transcripts were concentrated on the ooplasm periphery. In testicular tissues, the intensively expressed TSHβ cells were found to be spermatogonia and spermatocytes in the spermatogenic cysts. This is the first report of a TSHβ expressed in the gonads of any vertebrates in addition to the expected expression in the pituitary, and it expresses more transcripts in the gonads during sex reversal or testis than in the ovaries both in E. coioides and E. akaara. Importantly, the TSHβ identification in germ cells allows us to further investigate the functional roles and the molecular mechanisms in gametogenesis of groupers, especially in sex reversal and in spermatogenesis.